In general, windmills convert rotational energy of blades rotated by natural wind into electricity by an electric generator.
This is called wind power generation and they are classified into two types, according to the direction of rotational axis or the turbine, as HAWT (horizontal axis wind turbine) of which rotational axis is horizontal to the ground, and VAWT (vertical axis wind turbine) of which rotational axis is vertical to the ground.
And according to the sources of rotational force, the wind turbines can be classified into two types, as lift type and drag type. In lift type the blades are turned with the tangential components of lift forces generated on the blades, as the Darrieus type or the gyromill type. In drag type, the blades are turned by the drag forces, as a paddle type or Savonius type.
One of the problems in the case of drag type of VAWT is that, when the tip speed ratio (ratio of tangential speed of blade and natural wind speed, TSP) reaches 1, the increase in the wind speed cannot increase the rotational speed and as the result, the generating efficiency is low. But with the drag type, it is relatively easy to start the blade to move even with the lower wind speed.
In the lift type, when TSP is higher than 1, the aerodynamic efficiency of the turbine improves as the result of improved aerodynamic performance of the blades; however, when TSP is lower than 1, the aerodynamics performance of the windmill is worsened and the rotating moment of the blades are reduced. And because of the reduced rotating moment, it becomes difficult to start the blades to rotate from the stationary condition.
Therefore, there have been efforts to combine the advantages of drag type and lift type to make windmills to start at low wind speed and to have higher efficiency at the same time.
Such a hybrid type windmill is disclosed in Japanese Patent Publication No. 2003-314432.
The above mentioned windmill is achieved by integrally connect a Savonius windmill, that is a drag type, to the shaft of a Darrieus windmill, that is a lift type, such that rotation is started by the Savonius windmill and then high rotation speed is reached by the Darriues windmill.
However, in the above mentioned hybrid windmill, though the Savonius windmill can starts the DARRIEUS wind mill to rotate at low wind speed, as the rotating speed increases the Savonius windmill hinders the rotating movement of Darrieus windmill, and as the result, the efficiency of the whole system becomes lower than the case of Darrieus windmill only.